Why a 250 litre tank? What about a 175 litre tank?
175 litres of water weighs about the same as 2 large adults - so an estate car would be quite safe enough. Laid flat with battens to stop the tank moving forwards/backwards or sideways, there wouldn't be a problem if you're only using the estate for travelling round town (i.e. 30 mph or less).
You can nip home to re-fill, if necessary. It depends how many houses you intend to do WFP per day.
Get a van when your 'round' warrants it.
If you want to use the estate for longer/FASTER trips - empty the water out.
Mike